Heat the oven - Preheat the oven to 425°F and position a rack in the middle. I set two rimmed sheet pans nearby (lining with parchment is optional for easy cleanup).

Reduce the balsamic - I skip the mess and usually just use the store-bought balsamic glaze for drizzling, it's tasty as the reduced balsamic. But if you don't have one or prefer to make it yourself then pour 1/2 cup balsamic vinegar into a small saucepan and bring it to a gentle simmer over medium heat. Cook, stirring occasionally, until syrupy, reduced by about half, and it coats the back of a spoon, 6–10 minutes. Then pour it into a small bowl to cool completely; it thickens as it stands and should be drizzleable, not stiff.

Roast the salmon - Place 12 ounces salmon on the second rimmed sheet pan, pat it dry, and rub it with 1 teaspoon olive oil. Season all over with kosher salt and black pepper, then roast until it flakes easily and reaches about 125–130°F in the center on an instant-read thermometer if using, 8–12 minutes depending on thickness. Let it rest 5 minutes so it stays juicy.

Slice the baguette - Slice 1 baguette on a bias into 1/2-inch-thick slices (about 20). Arrange the slices in a single layer on a rimmed sheet pan.

Toast the crostini - In a small bowl mix together 3 tablespoons olive oil and the minced garlic and brush both sides of the bread using a pastry brush. I bake until lightly golden at the edges and crisp, 6–8 minutes, flipping halfway. Then transfer the toasts to a rack to cool so the bottoms stay crisp.

Flake the salmon - If the salmon is skin-on, lift off and discard the skin. You can cut it into cubes with a knife or use a fork to flake the salmon into bite-size pieces, keeping the texture light instead of mashed.

Prep the tomatoes - White roasting the salmon and toasting the crostini, seed and finely dice 2 Roma tomatoes (about 1 cup) and the onion.

Mix the topping - In a mixing bowl, I combine the tomatoes, chopped basil, chopped onion. I finely grate 1 teaspoon lemon zest with a microplane, then add it along with 1 tablespoon lemon juice, 1 tablespoon olive oil, 1/4 teaspoon kosher salt, and black pepper to taste. I toss, taste, and adjust with a pinch more salt and pepper if needed. Set the bowl aside.

Assemble the crostini — Right before serving, top each crostini with a generous pinch of flaked salmon and a spoonful of the tomato–herb topping. Drizzle the balsamic glaze over the top to lightly stripe each piece. Serve immediately for maximum crunch.